In a wireless device such as a mobile phone, an oscillator is required for generating a reference frequency that may be used by a wireless transceiver for transmitting and receiving wireless signals. Various types of oscillators may be used such that the reference frequency is generated with sufficient accuracy. Accordingly, oscillators have been made which compensate for sources of error in the generated reference frequency. One example of such an oscillator is a temperature compensated voltage controlled crystal oscillator (TCVCXO). However, the free running frequency of a TCVCXO may not be accurate enough in certain situations. Accordingly, a closed loop frequency compensation technique is typically utilized to fine-tune the generated frequency when the handset has acquired the signals from the wireless network, with the assumption that, the frequency of the incoming signals from a wireless network is accurate.
Closed-loop frequency compensation methods rely on some sort of phase locked loop (PLL) or automatic frequency control loop to adjust the local frequency source according to an external accurate frequency source. Wireless networks such as CDMA2000 networks have a very accurate reference frequency which is locked to a GPS system, and in turn is locked to the atomic frequency standard. Other wireless networks such GSM/GPRS, TDMA, etc also have a fairly accurate reference frequency although not to the degree of a CDMA2000 network. However, it is costly to use a closed loop frequency compensation method at all times for a variety of reasons. For instance, it takes time to accurately lock to a high frequency source. Also, before a frequency lock is obtained, the local frequency generator of the wireless device is still inaccurate. In addition, when the wireless device is working with another signal source, the local frequency generator may not be suitable for use with a closed loop compensation scheme to achieve a desired reference frequency accuracy since the other signal source may not have superior frequency accuracy (e.g. 802.11 WLAN). Accordingly, the wireless communications device can lock to the other signal source, but cannot achieve the required absolute frequency accuracy after tuning back from the other signal source. In another example, the other signal source may have a very accurate reference frequency, (e.g. it may obtain the reference frequency via GPS), but if the wireless communications device only tracks the other signal source for a very short period of time, such as 2 seconds for example, frequency tracking to the other signal source is difficult to establish. In both cases, there is also a power cost associated with relying on the external reference frequency since monitoring the other signal source requires additional power consumption by the wireless communications device.
Further, it has been found that the closed loop frequency compensation technique cannot help in the following cases:                1. During the initial acquisition of the wireless signal from the wireless network after the transceiver of the wireless device has been turned on after being off for an extended period of time;        2. During the initial re-acquisition of the wireless signal from the wireless network after the transceiver has “woken up” from a sleep state;        3. While the transceiver has tuned away from the wireless network to receive signals from other sources, and in this state the closed loop compensation technique is not functional; and,        4. At the initial period to reacquire the signal from the wireless network after the transceiver tunes back from the state described in case 3.Such cases require precise and quick generation of the reference frequency without closed loop compensation. This is because closed loop frequency compensation requires time to lock into the desired reference frequency and reach a steady state value for the reference frequency.        